Selectivity improvement in oxygen delignification and bleaching of lignocellulose pulp using singlet oxygen

ABSTRACT

A process to enhance delignification or bleaching in chemical or mechanical pulps comprising generation of singlet oxygen as a separate step and subsequent transport of singlet oxygen to pulp to effect bleaching or brightening of the pulp.

FIELD OF INVENTION

The present invention relates to a process for delignification andbleaching of lignocellulose pulp and improving selectivity by means ofwhich a stream of molecular oxygen is treated with visible light and achemical agent generating singlet oxygen and then applying the stream ofsinglet oxygen to pulp during the pulping procedure.

BACKGROUND OF INVENTION

Oxygen delignification is the use of molecular oxygen (i.e., ordinaryoxygen) and alkali to remove a substantial fraction of lignin inunbleached chemical pulp before conventional bleaching. Oxygen bleachingis considered synonymous with oxygen delignification. The process isusually conducted with molecular oxygen under high pressure and atelevated temperatures. Molecular oxygen is usually applied to kraftunbleached pulps but can also be used for other chemical pulps. Oxygendelignification is now a commercial process, and a number of patentshave been published. These patents describe the use of molecular oxygen,arrangements, number of stages, process conditions—time and temperature,and addition of alkali/oxygen charge.

Oxygen bleaching typically is not applied to mechanical pulp. Ingeneral, mechanical pulps are bleached with a reducing agent, such assodium hydrosulfite (sodium dithionite) or hydrogen peroxide, or acombination thereof.

Oxygen delignification involves a rather complex chemistry. Underalkaline conditions at high pressure, oxygen reacts to form both freeradicals and anions, including superoxide radicals, peroxide anions, andhydroxyl radicals. These species interact with lignin in a complexfashion; however they can also degrade cellulose causing yield loss anddecreased pulp strength.

Degradation of the cellulose resulting in yield loss can be classifiedinto two categories: random chain cleavage and the “peeling” reaction.Random chain cleavage which may occur at any point along the polymericchain is more significant in oxygen delignification due to the amount offree radicals present. “Peeling” causes monosaccharide units at the endof the chain to be attacked and successively removed. Peeling isgenerally not an issue in kraft pulp since the acidic chain ends havebeen converted to the more stable oxidized form. However, when randomchain cleavage is excessive, peeling can become a problem since two newchain ends are formed which have a reducing end group

The chemistry of oxygen is unique. Molecular oxygen has a normal (lowestenergy) configuration that is a triplet state containing two unpairedelectrons, i.e. a free radical. However, the first excited state ofoxygen molecules is a singlet state. The lifetime of singlet oxygen insolution is in the microsecond range (3 μsec in water) but is extremelylong lived (72 minutes) in the gaseous phase. Singlet oxygen reactsselectively with carbon-carbon double bond either by the Diels-Alderreaction or by the ene reaction. Native lignin or modified lignins aftervarious pulping processes contain functional groups such as olefinic,phenolic, and non-phenolic groups, which can potentially react withsinglet oxygen (Frimer, Singlet O ₂ , Volume IV—Polymers andBiomolecules, pp. 21-24, CRC Press, 1985).

Prior art on singlet oxygen chemistry in the pulp and paper industrygenerally falls into one of the following categories:

-   -   1. Processes that generate singlet oxygen in situ by using        chlorine-containing compounds in combination with        oxygen-containing compounds (U.S. Pat. No. 4,008,120; Szabo et        al., Cellulose Chem. Technol., 28, 183, 1994). In these        processes, bleaching can occur from both the chlorine-containing        compounds (e.g., hypochlorite) and from the singlet oxygen.    -   2. Processes of direct irradiation of the lignocellulosic pulp        mixture in a pH between 8 and 13 with ultraviolet light in the        presence of oxygen (Turner, U.S. Pat. No. 4,294,654). Turner's        Example 1 suggests that a photosensitizer is not required for        his process, and in fact the reported results are the same with        or without photosensitizer in his process. Direct irradiation of        pulp using specific ultraviolet light at 300-420 nm to        facilitate bleaching has been previously reported in U.S. Pat.        No. 2,161,045.    -   3. Processes using corona (or other) discharge methods to        produce “active” gas, which is used for delignification and        bleaching of chemical and mechanical pulp at a consistency of        15-95% (U.S. Pat. No. 3,806,404, Liebergott et al.). The use of        activated oxygen with softwood was disclosed by Liebergott in        Example 1, but the decrease in Kappa number was modest. As        pointed out by Turner (U.S. Pat. No. 4,294,654), the activated        nitrogen was found by Liebergott to be extremely effective,        while activated oxygen under the same reaction conditions was        far less effective.    -   4. Processes using pure model lignin compounds using singlet        oxygen generated via photo-oxygenation, primarily related to        color reversion of cellulosics. (G. Gellerstedt, K. Krinstad,        and E. L. Lindfors, “Singlet Oxygen Oxidation of Lignin        Structures” in Singlet Oxygen reaction with organic compounds        and polymers, eds. B. Rånby and J. F. Rabek, pp. 302-310, John        Wiley & Sons, 1978)

Liebergott dealt with the bleaching of chemical pulps by the use of ahigh consistency gaseous phase bleaching process. The high consistencyis defined as 15-95% (i.e., 5-85% water). In their own words, the“‘active’ or ‘electronically excited’ singlet oxygen or high energyoxygen triplet is . . . produced by passing the respective gas . . .through an electrode-less discharge, or through a microwave discharge,or through a condensed pulsed discharge or by a ‘plasmajet.’” The use ofelectronically excited states of oxygen with softwood is disclosed byLiebergott in Example 1-(1), where ‘oxygen, at a rate of 0.009liter/min, at 55 kPa, was passed through a corona discharge, where itwas acted upon by a primary potential of 120 volts.’ However, theyindicated that gaseous mixture formed in their process is not singletoxygen, but ozone. Furthermore, as pointed out by Turner, the data inTable I in Liebergott indicate that activated oxygen [generated perLiebergott's process] was only marginally effective in delignifying andbleaching lignocellulosic pulps. Reference to Table I of the Liebergottpatent indicates that subsequent to bleaching the Kappa number of thepulp was 22.6, which represents a reduction of only 1.4 units, whichtranslates into a percentage reduction of only 5.8%.

Finally, Szabo suspended pulp in an aqueous solution of H₂O₂ and thenadd NaClO to the solution. H₂O₂ and NaClO react chemically to generateabout 10% singlet oxygen. This is a single-step process. Furthermore,there is no assurance that residual H₂O₂ or NaClO may not carry out somebleaching. Thus, the observed bleaching results may not necessarily bedue to singlet oxygen. In addition, their data (Table 2) did not showany significant increase in activity of H₂O₂/NaClO versus NaClO alone.In fact, NaClO alone shows higher delignification and higher selectivitythan H₂O₂/NaClO.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed is a novel process to enhance delignification or bleaching inchemical or mechanical pulps comprising photochemical generation ofsinglet oxygen in a separate step and subsequent transport of thesinglet oxygen to pulp to achieve bleaching or brightening. Preferably,we enhance bleaching of lignocellulose pulp in the oxygendelignification process by treating a stream of molecular oxygen withvisible light and a chemical agent converting molecular oxygen intosinglet oxygen and applying the resulting stream of singlet oxygen topulp during the pulping procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a laboratory system using ambient pressurefor producing singlet oxygen that may be used to enhance delignificationor bleaching in accordance with one embodiment of the invention.

FIG. 2 is a block diagram of a laboratory system using high pressure forproducing singlet oxygen that may be used to enhance delignification orbleaching in accordance with one embodiment of the invention.

FIG. 3 is a block diagram of a possible industrial scale system usinghigh pressure with a five chamber device for producing singlet oxygenthat may be used to enhance delignification or bleaching in accordancewith one embodiment of the invention. More or less chambers can also berealized.

FIG. 4 is a block diagram of a portion of a possible two-stageindustrial system using high pressure for producing singlet oxygen thatmay be used to enhance delignification or bleaching in accordance withone embodiment of the invention. The same setup with one or multiplestages can also be realized.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a process for the delignification orbleaching of lignocellulosic pulp comprising the steps of a) providingan aqueous or non-aqueous mixture of pulp, b) providing a stream ofmolecular oxygen, reacting the molecular oxygen in the presence ofvisible light and at least one chemical agent photochemically convertinga portion of the molecular oxygen to singlet oxygen and c) subsequentlycontacting pulp or pulp mixture with the singlet oxygen stream.

The present invention also provides for pulp or paper product producedusing singlet oxygen.

The conversion of raw lignocellulosic material to unbleached and thenbleached pulp requires an extremely complex and intricate series ofchemical reactions and physical processing, usually requiring two ormore stages in which different reactions are involved. The first isreferred to as pulping, and the second as bleaching. Both, however,include delignification.

Chemical or mechanical treatments of wood or other plant materials areused to generate pulp, which is then made into paper products. Inchemical pulping, a “digestion” process is used (e.g., kraft or sulfite)where solutions of various chemicals dissolve or modify lignin,non-fibrous materials and other impurities to liberate pulp. Inmechanical pulping, a mechanical grinding motion separatescellulose-containing fibers from a wood matrix. One method of mechanicalpulping uses a disc refiner, which typically has two metal plates whereat least one of the plates is rotating at high speed; the wood chips isfed into the refining area and are broken down at high pressure andtemperature between the center (eye) of the refiner and the otherrefiner zones. Another method (called “groundwood”) is to press the woodagainst the face of a rapidly revolving grindstone, where the abrasiveaction tears the fibers from wood matrix. Many variations of thesemethods are known, e.g., semi-chemical, thermo-mechanical, andchemi-thermo-mechanical pulping.

“Oxygen delignification” is the use of molecular oxygen and alkali toremove a substantial fraction of lignin in unbleached chemical pulpbefore conventional bleaching. Oxygen bleaching is considered synonymouswith oxygen delignification. The process is usually conducted withmolecular oxygen under high pressure and at elevated temperatures.Molecular oxygen is usually applied to unbleached kraft pulp but canalso be used for other chemical pulps.

Oxygen delignification as single-stage or multiple-stage process is wellknown in the art. Oxygen bleaching typically is not applied tomechanical pulp. The present invention can be used at one of more stagesin a oxygen delignification process.

In general, mechanical pulps are bleached or brightened with a reducingagent, sodium hydrosulfite (sodium dithionite) or hydrogen peroxide, ora combination of sodium hydrosulfite and hydrogen peroxide.

The lignocellulosic pulp employed in the present process can be preparedfrom any lignocellulose-containing material derived from natural sourcessuch as, but not limited to, hardwood, softwood, gum, straw, bagasseand/or bamboo by various chemical, semichemical, mechanical orcombination pulping processes. Chemical and semichemical pulpingprocesses include, but not limited to kraft, modified kraft, kraft withaddition of sulfur and/or anthraquinone, and sulfite. Mechanical pulpingprocesses includes, but not limited to stone groundwood, pressurizedgroundwood, refiner mechanical, thermo-refiner mechanical, pressurerefined mechanical, thermo-mechanical, pressure/pressurethermo-mechanical, chemi-refiner-mechanical, chemi-thermo-mechanical,thermo-chemi-mechanical, thermo-mechanical-chemi, and long fiberchemi-mechanical pulp. Handbook for Pulp and Paper Technologist, ed. G.A. Smook (Atlanta, Ga., TAPPI Press, 1989) describes both chemical andmechanical pulping.

The term “oxygen” or “diatomic oxygen” is considered synonymous withmolecular oxygen (i.e., ordinary oxygen). Oxygen has an electronconfiguration of an open-shell triplet ground state (³O₂) having twounpaired electrons occupying two degenerate molecular orbitals and isone of two major components of air.

The term “singlet oxygen” refers to the first electronically excitedstate of oxygen (¹O₂) (also known as singlet delta—a¹Δ_(g)) in which allthe electron spins are paired. Singlet oxygen has a higher energy thantriplet ground state oxygen. Singlet oxygen has a limited lifetime insolution (microsecond range) and gaseous phase (less than 2 hours).

The present invention provides for a method to generate singlet oxygenthat consists of the steps of providing a molecular oxygen source, avisible light source, a chemical agent wherein the molecular oxygensource provides a stream of molecular oxygen into a chamber or a seriesof chambers containing the chemical agent, contacting the molecularoxygen with the chemical agent in the presence of visible light from thelight source.

The singlet oxygen employed in the process of the invention is generatedwith visible light and a chemical agent. Molecular oxygen in thepresence of a photosensitive chemical agent is exposed to visible lightin a chamber resulting in the generation of singlet oxygen. The singletoxygen stream is then brought into contact with the pulp mixture. Thesinglet oxygen reacts with the pulp resulting in delignification.

Visible light employed in the process of the invention has a wavelengthbetween 400 nm to 700 nm. Sources of visible light are selected from thegroup consisting of, but are not limited to, halogen lamps, tungsten orfluorescent lamps, light emitting diodes (LED), lasers, or combinationthereof. Any light source with filters to limit the wavelength tovisible light may be used.

The method of the invention provides increased selectivity of the oxygendelignification process by converting a portion of the molecular oxygento singlet oxygen. Selectivity is defined as the ratio of the intrinsicviscosity of the pulp versus the Kappa number of the pulp. Intrinsicviscosity is a measure of cellulose molecular weight and cellulosedegradation. Kappa number is a well known indicator of the lignincontent or bleachability of pulp. It indicates the amount of bleachneeded during digestion of wood pulp to obtain a pulp with a given levelof whiteness. A higher selectivity indicate a better process, and moredesirable pulp product. The present invention provide for an improvedselectivity based on molecular weight data of at least about 9%.

Chemical agents useful in the present invention include, but are notlimited to, photosensitizer dye, pigment, aromatic hydrocarbon, coenzymeor biochemical, metallic salt, and transition metal complex.Photosensitizer dyes include, but are not limited to, methylene blue,rose bengal, eosin, crystal violet, and acridine orange. Pigments usefulin the present invention include, but are not limited to, chlorophyll,hematoporphyrines, and flavin. Aromatic hydrocarbons useful in thepresent invention include but are not limited to rubrene and anthracene.Coenzymes or biochemical agent useful in the present invention include,but not are limited to, pyridoxals and psoralenes. Metallic salts usefulin the present invention include, but are not limited to, cadmiumsulfide and zinc sulfide. Transition metal complexes useful in thepresent invention include, but are not limited to, ruthenium andbipyridine. The chemical agents useful in the present invention canabsorb light in the 380-900 nm, preferably 400-700 nm range (visiblelight). The chemical agent can be affixed to gas filters, glass beads,wire mesh, or a catalytic bed.

The photosensitizer dye exhibits fluorescence and phosphorescencereflecting two separate electronically excited states, namely, thesinglet state and the triplet state. The singlet state is produced firstby the absorption of light, but it has a short lifetime, decaying byfluorescence to the ground state and by electronic intersystem crossingto the triplet state. The triplet state of these photosensitizers decaysto the ground state at a slower rate. The most effectivephotosensitizers have a high quantum yield of a long-lived tripletstate.

The conversion of molecular oxygen into singlet oxygen is dependent onthe solvent and chemical agent used and measured as quantum yield. Forexample, quantum yield for singlet oxygen formation Φ(¹O₂)=0.76 for rosebengal in methanol. The quantum yield for the formation of the excitedtriplet state of rose bengal is 0.76. Therefore, all triplet oxygen(molecular oxygen) is converted into singlet oxygen. It is preferredthat a least 10% of the molecular oxygen is converted into singletoxygen. The conversion of molecular oxygen into singlet oxygen can befrom 5% to 100%. Preferably the conversion of molecular oxygen intosinglet oxygen is 10% to 100% and most preferably the conversion ofmolecular oxygen into singlet oxygen is 20% to 100%. The supply ofmolecular oxygen and/or air into the chamber containing the chemicalagent(s) and visible light must contain less than 3.5 molar ppm of water(H₂O).

In a kraft oxygen delignification system, singlet oxygen may begenerated and added either to the “mixer” before to the first oxygenreactor and/or added in the main charge position before the first oxygenreactor in the reactor system and separate from the steam line.

In mechanical pulps, the singlet oxygen is applied to the mechanicalpulp during the pulping process at the eye of a mechanical pulp refineror through the dilution water for stone ground wood. It can also beapplied at other points in the pulping process such as at the storagetanks, at a washing step, at a beaching step. Typical mechanical pulpingconditions are well know in the art.

The consistency of solids of the pulp mixture can be from 0.5% to 28%,based upon the weight of oven-dried pulp. The consistency can be as highas 28% or as high as 20% or preferable as high as 14%. The consistencyis at least 0.5%, or at least 3% or at least 5%, or at least 8%.Preferably the consistency of the pulp mixture is 5% to 28% and, mostpreferably, the consistency of the pulp is 9% to 14%. The pulp mixtureuseful in the present invention comprises the pulp, water and/or anorganic solvent. The organic solvent is selected from the groupconsisting of acetone, acetonitrile, ethanol, methanol, isopropanol,acetic acid or combination thereof. The ratio of water to organicsolvent is about 100:0 to about 0:100, preferably 100:0 to 80:20, andmore preferably 100:0 to 90:10.

The chemical pulping consists of several unit operations, one of whichis oxygen delignification. In the oxygen delignification process forchemical pulp, the caustic consumption ranges from 0 to 24 kg/toncorresponding to a pH range of 7 to 12, preferable at least 8.5; thereactor temperature ranges from 20° C. to 160° C. or 25° C. to 160° C.,preferably 80° C. to 150° C., and more preferably 85° C. to 125° C.; andthe pressure ranges from atmospheric pressure to 1 MPa, preferably0.1-1.0 MPa, and more preferably 0.5-0.8 MPa. Preferably the pressure isat least 0.1 MPa. It is under these conditions in the oxygendelignification process of chemical pulp that the singlet oxygen wouldbe contacted with the pulp. Generally in an oxygen delignificationprocess the consumption of molecular oxygen can be from 1 to 100 kg perton pulp, preferably from 5 to 50 kg per ton pulp, and most preferablybetween 5 to 25 kg per ton pulp.

In the present invention, oxygen is treated in a separate stream in achamber via a combination of visible (non-UV) light and a chemicalagent, which is then applied to the pulp. This is in contrast to theprior patent by Turner, which uses UV or corona discharge. UV lightgenerates more reactive and less selective species such as free radicalswhich can cause degradation to the pulp as compared to visible light; UVlight is more expensive to produce then visible light; and UV light ismore hazardous to the skin and to human eyes and is more difficult toinstall in a pulp or paper mill than is visible light. The presentinvention which uses visible light is therefore more beneficial than theprocess of U.S. Pat. No. 4,294,654. Turner suggests that aphotosensitizer is not required for his process; see Example 1 in Turnerwhere the reported results are the same with or without photosensitizerin his process. In the present invention, chemical agent/photosensitizeris a required element of the process. In contrast to Turner's process,the present invention never applies the light energy directly to thepulp, thereby obviating the absorption of light by cellulose asdescribed in U.S. Pat. No. 2,161,045.

In contrast to the results obtained by Liebergott using activated oxygenproduced by discharge methods having consistencies from 15-95%, it hasnow surprisingly been found, as shown in the present invention, that ahighly effective process for the delignification and bleaching of lowconsistency pulps is achieved by using visible light and a chemicalagent to specifically generate singlet oxygen in a mild reaction andexposing the resulting singlet oxygen to the pulp. The data set forth inthis invention for unbleached kraft softwood pulp in Table 1 formethylene blue and Table 3 for rose bengal illustrate the selectivityimprovement of oxygen delignification in the presence of singlet oxygenof the present invention compared to oxygen delignification withmolecular oxygen. For methylene blue, the Kappa number decreases from22.7 with molecular oxygen to 20.2 with singlet oxygen showing greaterdegree of bleaching and bleachability with singlet oxygen. A decrease inKappa number is desirable. As shown in Table 1 for methylene blue, theintrinsic viscosity drops drastically decreases from 778 g/mL for thestarting pulp to 668 g/mL during oxygen delignification with molecularoxygen whereas the intrinsic viscosity much smaller decrease to 773 g/mLduring oxygen delignification with singlet oxygen. Molecular oxygencauses more cellulose degradation than singlet oxygen. The process inthis invention produces significant selectivity improvement of 23.5%with methylene blue as the chemical agent as shown in Example 1 and of36.9% with rose bengal as the chemical agent as shown in Example 3. Theselectivity improvement obtained with this invention provides pulpand/or paper prepared from such pulp with higher degree ofpolymerization which is evidenced by a higher viscosity of thecellulose, at the same Kappa number.

Unbleached mechanical pulp as shown in Example 11 and Table 11 of thisinvention, surprisingly showed selectivity improvement was found at9.49% with singlet oxygen as compared to molecular oxygen. The Kappanumber for oxygen delignification with molecular oxygen shows anincrease from 57.9 to 67.6 whereas oxygen delignification with singletoxygen shows a decrease in Kappa number from 57.9 to 52.0. Whereas theweight average molecular weight as measured by size exclusionchromatography for the starting pulp was 689,000 dalton, for oxygendelignification with molecular oxygen was 621,000 dalton and for oxygendelignification with singlet oxygen was 523,000 dalton.

The present invention 1) uses visible light and a chemical agent (aphotochemical reaction) to produce singlet oxygen, and 2) carries outthe process in two steps, separately generating the singlet oxygen andthen transporting the singlet oxygen to pulp.

Examples shown below are used for illustration but are not limiting.

EXAMPLES

Examples 1-12 and the corresponding Tables 1-12 show the followingapplication of singlet oxygen:

-   -   Effect of pressure—ambient pressure (Example 1) and 40 psig        (Examples 2-12).    -   Effect of chemical agent—methylene blue (Examples 1 & 2), rose        bengal (Examples 3, 6-12), eosin Y (Example 4), and mixture of        chemical agents (Example 5)    -   Effect of manifold length—6.25 cm (Example 6) and 12.5 cm        (Examples 2-5, 7-12)    -   Effect of gas—oxygen (Examples 1-12) and compressed air (Example        3)    -   Effect of additives in combination with singlet oxygen—guar        (Example 7), peroxide (Example 8)    -   Effect of light source—halogen 500 W (Examples 1-8, 10-12) and        fluorescent light (Example 9)    -   Effect of furnish—softwood (Examples 1-9, 12), hardwood (Example        10), and mechanical pulp (Example 11).    -   Effect of pulp consistency—low, medium, and high (Example 12).

General Procedure Employed in Examples

Size Exclusion Chromatography in DMSO:

The procedure for preparing cellulose samples for SEC indimethylsulfoxide (DMSO). Cellulose was made soluble by converting it tothe methylol cellulose derivative. Approximately 75 mg of sample and 1.6g of paraformaldehyde was added to 56 g of DMSO containing 1.0% LiCl and500 ppm BHT. This mixture was heated with stirring at 110° C. for 40minutes. This solution is referred to as the cook solution.Paraformaldehyde decomposes into formaldehyde during the samplepreparation step, which subsequently reacts with cellulose to form thederivative which is soluble in DMSO.

The chromatographic conditions used were:

Chromatograph: Waters Alliance 2000 GPCV

Primary Detector Waters Differential Refractometer, 45° C.

Secondary Detector Waters Single Capillary Viscometer, 45° C.

Columns: 1 PL-Gel Mixed A

Column Temperature: 45° C.

Mobile phase: DMSO with 0.5% LiCl and 3.0% Formalin

Flow rate: 0.2 ml/min

Run Time: 100 min

Sample Concentration: 0.06 wt %

Injection volume: 325 μl

Internal standard: THF

M_(W) Calibration Standards: Pulullan

The analyte solution was prepared by adding 9.6 g of mobile phase to 0.4g of cook solution. The analyte solution was filtered through 0.45 μmnylon filter. The refractive index chromatograms were processed using apullulan calibration and Waters Empower software. The weight-averagemolecular weight (Mw) is sued in the calculation of selectivity.

TAPPI brightness index and CIE L*a*b* color space values of compressedcellulose sheet samples were determined by standard method. (TAPPIMethod of Test T452—“Brightness of pulp, paper & paperboard (directionalreflectance at 457 μm)”. For each sample, five readings were taken onthe sheet surface using X-Rite 532 spectrodensitometer. The observerangle for L*a*b* measurement is 2°.

Kappa number: Kappa number of the pulp was determined according to TAPPIStandard Method T236 cm-85.

Intrinsic viscosity: The intrinsic viscosity of the pulp was determinedaccording to ISO 5351—“Pulps—Determining of limiting viscosity number incupriethylenediamine (CED) solution”.

Selectivity: Selectivity is defined as the ratio of intrinsic viscosityversus Kappa number. Selectivity is used to assess the benefits of pulpbleaching or delignification since the viscosity is related to thecellulose molecular weight and Kappa number provides a measure of thedegree of bleaching and bleachability. In Tables 2-12, the selectivitylisted is calculated from the ratio of intrinsic viscosity versus Kappanumber and also from the ratio of SEC weight-average molecular weightversus Kappa number.

-   -   Selectivity=Intrinsic Viscosity/Kappa number    -   or=Weight-average molecular weight/Kappa number    -   Selectivity Improvement=    -   (Singlet Oxygen Selectivity Datum−Control Molecular Oxygen        Selectivity Datum)/Control Molecular Oxygen Selectivity        Datum*100%    -   Selectivity Improvement can be calculated based on the data from        the intrinsic viscosity or from the data from the molecular        weight.

Example 1

The equipment included a three-necked round bottom flask equipped with amechanical stirrer, a condenser with gas bubbler, gas inlet with aU-tube (or reduced ends tube) filled with methylene blue prior toentering the round bottom flask, and a 500 W portable halogen worklightsource (Stanley Tools, Manufacturer No. XG-1009) as shown in FIG. 1. Theflask was charged with 110.82 g deionized water, 3.0 g sodium hydroxide(NaOH), 0.18 g magnesium sulfate (MgSO₄) and 6.0 g softwood pulp with astarting Kappa number of 54.4. Softwood pulp was added after the sodiumhydroxide and magnesium sulfate completely dissolved. The reaction flaskwas heated to reflux (100° C.) for four hours. Molecular oxygen flowsfrom the oxygen tank and is treated in a separate stream in a chambervia a combination of visible light and a chemical agent, which is thenapplied to the pulp mixture. The flow rate of the molecular oxygen wasbetween 110 to 140 mL/min and more typically 125 mL/min as measured byAgilent flowmeter ADM 2000 (Agilent Technologies, Inc., Wilmington,Del.). For the molecular oxygen experiments, the methylene blue wascovered with aluminum foil. The light from a halogen lamp (500 W) wasapplied during the singlet oxygen experiments. Initial experiments wereperformed at atmospheric pressure.

Unbleached, prehydrolyzed kraft softwood (Canadian spruce from J. D.Irving, St. John, NB, Canada) pulp was used as received after drying.Its starting Kappa number was 54.4, its starting intrinsic viscosity 778mL/g, and its weight-average molecular weight 984,000 daltons. SampleI.D. #1A and 1B were obtained using methylene blue as thephotosensitizer.

TABLE 1 EFFECT OF AMBIENT PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING METHYLENE BLUE OR ROSE BENGAL AS THEPHOTOSENSITIZER. Intrinsic Mw Selectivity Selectivity Sample ViscosityKappa (g/mol) Selectivity Selectivity Improvement Improvement I.D.Description [η], mL/g Number SEC from [η] from Mw from [η] from Mw 1AMolecular 668 22.7 978,000 29.4 43,100 — — Oxygen 1B Singlet 733 20.2896,000 36.3 44,300 23.5% 2.78% Oxygen

It is apparent from Example 1—Table 1 of the present invention that anon-optimized, ambient pressure system can achieve significantselectivity improvement in the presence of singlet oxygen generated bypassing molecular oxygen through a chamber with a chemical agent andvisible light to produce singlet oxygen which is then applied to thepulp. The Kappa number decreases from 22.7 with molecular oxygen to 20.2with singlet oxygen showing greater degree of bleaching andbleachability with singlet oxygen. A decrease in Kappa number isdesirable. Table 1 demonstrates that with molecular oxygen the intrinsicviscosity drops drastically from 778 g/mL to 668 g/mL whereas, withsinglet oxygen, the intrinsic viscosity drops only slightly to 773 g/mL.Molecular oxygen causes more cellulose degradation than singlet oxygen.The process produces significant selectivity improvement of 23.5% (asmeasured by the ratio of the intrinsic viscosity versus Kappa number).

In Examples 2-12, the following high-pressure experimental apparatusconsists of compressed oxygen tank which feeds pressured oxygen into aone-piece pressure manifold (Ace Glass, Vineland, N.J., Cat. No. 6448)packed with glass wool, Drierite® anhydrous calcium sulfate (W. A.Hammond Drierite Company LYD, Xenia, Ohio), and borosilicate glass beads(size 3 mm) coated with the photosensitizer dye up to 12.5 cm length.The manifold feeds pressurized singlet oxygen into a 300-mL reactor witha programmable controller (Parr Instrument Co., Moline, Ill., Model No.4561 and 4843) to a back pressure regulator (Swagelok, HuntingdonValley, Pa.). A safety shield (RAD-GARD®, Instruments For Research andIndustry, Inc., Cheltenham, Pa.) was placed between the 500 W Stanley®portable halogen worklight source and the pressure manifold as shown inFIG. 2. Molecular oxygen flows from the oxygen tank through the chemicalagent chamber and then into the pulp mixture. The flow rate of themolecular oxygen was between 110 to 140 mL/min and more typically 125mL/min as measured by Agilent flowmeter ADM 2000 (Agilent Technologies,Inc., Wilmington, Del.). For the molecular oxygen experiments, thechemical agent was covered with aluminum foil. The light from a halogenlamp (500 W) was applied during the singlet oxygen experiments.

In Example 9 (one of the singlet oxygen experiments), two Craftsmang 23Wfluorescent worklight sources were used.

Example 2

The Parr reactor was charged with 180 g deionized water, 19.31 gsoftwood pulp (18.0 g oven dried pulp), and 0.0072 g NaOH. Softwood pulpwas added after the sodium hydroxide and magnesium sulfate completelydissolved. The reaction flask was heated to 100° C. at 275 kPa for 90minutes. The one-piece pressure reactor manifold (Ace Glass, Vineland,N.J.) was packed with glass wool, Drierite® anhydrous calcium sulfate,and glass beads coated with the methylene blue photosensitizer. For thecontrol molecular oxygen experiments, the methylene blue photosensitizerwas covered with aluminum foil. The light from a halogen lamp (500 W)was applied during the singlet oxygen experiments. The pulp consistencywas 10%.

After a specified time, the reacted pulp was isolated on a Buchnerfunnel with a Whatman #41 filter paper under aspirator vacuum. The pH ofthe filtrate was measured between 7.9 and 9.4. The pulp was rinsed bythe addition of about 500 mL D.I. water to the funnel with aspiratorvacuum. The pulp was rinsed again by the addition of about 500 mL D.I.water with aspirator vacuum. The pulp was then air dried overnightfollowed by vacuum oven drying at 40° C. for 2 hours. The amount ofMgSO₄ added for sample I.D. #2A and 2B was 0.0 g. The amount of MgSO₄added for sample I.D. #2C and 2D was 0.009 g.

TABLE 2 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING METHYLENE BLUE AS THE PHOTOSENSITIZER.Intrinsic Mw Selectivity Selectivity Sample Viscosity Kappa (g/mol)Selectivity Selectivity Improvement Improvement TAPPI I.D. Description[η], mL/g Number SEC from [η] From Mw from [η], % from Mw, % Brightness2A Molecular 585 24.0 905,000 24.3 37,700 — — 26.8 Oxygen 2B Singlet 64724.1 106,000 26.8 44,000 10.3 15.8 25.8 Oxygen 2C Molecular 607 24.9842,000 24.4 33,800 — — 26.8 Oxygen 2D Singlet 646 23.3 952,000 27.740,800 13.5 20.6 26.9 Oxygen

Table 2 demonstrates that using a high pressure experimental apparatus,the selectivity improvement is 10.3% without MgSO₄ and 13.5% with MgSO₄using methylene blue as the chemical agent. In the remaining tables,MgSO₄ was added in the molecular and singlet oxygen examples. MgSO₄ is aselectivity agent as described in Example 7.

Example 3

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thatthe photosensitizer was rose bengal and compressed oxygen (or compressedair) was used as the gas. The amount of MgSO₄ added for sample I.D. #3A,3B, 3C, and 3D was 0.009 g.

TABLE 3 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER. IntrinsicMw Selectivity Selectivity Sample Viscosity Kappa (g/mol) SelectivitySelectivity Improvement Improvement TAPPI I.D. Description [η], mL/gNumber SEC from [η] from Mw from [η], % from Mw, % Brightness 3AMolecular 607 24.9 842,000 24.4 33,800 — — 26.8 Oxygen 3B Singlet 71121.3 930,000 33.4 43,700 36.9 29.4 26.9 Oxygen 3C Air - 646 27.8 943,00023.2 33,900 — — 24.6 M.O. 3D Air - 665 26.3 892,000 25.3 33,900  8.8 0.0 24.1 S.O. M.O. = molecular oxygen and S.O. = singlet oxygen

The data set forth for unbleached kraft softwood pulp in Table 3 forrose bengal illustrates the selectivity improvement of 36.9% (SampleI.D. 3B) for oxygen delignification in the presence of singlet oxygen ofthe present invention compared to oxygen delignification with molecularoxygen. The high selectivity improvement of 39.9% (based on the ratio ofintrinsic viscosity versus Kappa number) was obtained by a decrease inKappa number from 24.9 (Sample I.D. 3A) with molecular oxygen to 21.3(Sample I.D. 3B) with singlet oxygen showing greater degree of bleachingand bleachability with singlet oxygen. A decrease in Kappa number isdesirable. Table 3 also shows that the intrinsic viscosity drasticallydecrease from 778 g/mL for the starting pulp to 607 g/mL (Sample I.D.3A) during oxygen delignification with molecular oxygen whereas withsinglet oxygen the intrinsic viscosity had a much smaller decrease to711 g/mL (Sample I.D. 3B). Molecular oxygen causes more cellulosedegradation than singlet oxygen.

Example 4

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was eosin Y. The amount of MgSO₄ added for sample I.D.#4A and 8B was 0.009 g.

TABLE 4 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING EOSIN Y AS THE PHOTOSENSITIZER. Intrinsic MwSelectivity Selectivity Sample Viscosity Kappa (g/mol) SelectivitySelectivity Improvement Improvement TAPPI I.D. Description [η], mL/gNumber SEC from [η] from Mw from [η], % from Mw Brightness 4A Molecular607 24.9 842,000 24.2 33,800 — — 26.8 Oxygen 4B Singlet 548 22.3 883,00024.6 39,600 0.82 17.1 27.6 Oxygen

Example 5

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was a mixture of equal amounts of methylene blue androse bengal. The amount of MgSO₄ added for sample I.D. #5A and 5B was0.009 g.

TABLE 5 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING MIXTURE OF METHYLENE BLUE AND ROSE BENGAL ASTHE PHOTOSENSITIZER. Intrinsic Mw Selectivity Sample Viscosity Kappa(g/mol) Selectivity Selectivity Improvement Selectivity TAPPI I.D.Description [η], mL/g Number SEC from [η] from Mw from [η], % from MwBrightness 5A Molecular 607 24.9 842,000 24.4 33,800 — — 26.8 Oxygen 5BSinglet 647 23.2 873,000 27.9 37,600 14.3 11.8 25.1 Oxygen

Table 5 shows that when a mixture of methylene blue and rose bengal isused as the chemical agent, the selectivity improvements more closelymatches that of methylene blue.

Example 6

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose bengal at a length of 6.25 cm. The amount ofMgSO₄ added for sample I.D. #6A and 6B was 0.009 g.

TABLE 6 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING ROSE BENGAL (length = 6.25 cm) AS THEPHOTOSENSITIZER. Intrinsic Mw Selectivity Selectivity Sample ViscosityKappa (g/mol) Selectivity Selectivity Improvement Improvement TAPPI I.D.Description [η], mL/g Number SEC from [η] from Mw from [η], % from MwBrightness 6A Molecular 607 24.9 842,000 24.4 33,800 — — 26.8 Oxygen 6BSinglet 645 22.8 873,000 28.3 38,300 16.0 11.8 28.7 Oxygen

Table 6 demonstrates decreasing the surface area of the rose bengal(chemical agent) changes the selectivity improvement proportionally.

Example 7

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose bengal. Guar was also added. The amount ofMgSO₄ added for sample I.D. #7A and 7B was 0.009 g and the amount ofguar added was 0.19 g.

TABLE 7 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER - GUARADDITION. Intrinsic Mw Selectivity Selectivity Sample Viscosity Kappa(g/mol) Selectivity Selectivity Improvement Improvement TAPPI I.D.Description [η], mL/g Number SEC from [η] from Mw from [η], % from Mw, %Brightness 7A Molecular 639 24.0 885,000 26.6 36,900 — — 25.1 Oxygen 7BSinglet 670 20.4 860,000 32.8 42,200 23.3 13.5 28.0 Oxygen

Examples 7 and 8 show the effect of singlet oxygen in the presences oftwo different selectivity agents—guar and hydrogen peroxide. Selectivityagents are generally chemicals that are added during oxygendelignification. The selectivity agents reported in prior art aregrouped into five categories: (1) oxidizing agents, such as, but notlimited to hydrogen peroxide, chlorine, chlorine dioxide; (2) complexingagents that either remove or inactivate transition metal ions, such as,but not limited to magnesium sulfate (MgSO₄), sodium or magnesiumgluconate, muconic acid, ethylenediaminetetraacetic acid (EDTA) and itssalts, diethylenetriaminepentaacetic acid (DTPA) and its salts; (3)radical scanvengers which reduce the amount of free radicals presentsuch as, but not limited to, muconic acid, sodium or magnesiumgluconate, sodium 1-hydroxyethylidene-1,2-diphosphonate (HEDP); (4)cellulose protective materials which absorb onto the cellulose fiberssuch as, but not limited to, magnesium sulfate (MgSO₄), phenol, phenoland magnesium sulfate, guar and hemicellulose; and (5) others whichinclude, but not limited to enzymes, surfactants, organic solvents(organosolv process), and spent liquor.

In the presence of guar as shown in Table 7, the selectivity improvementis 23.3% (Sample I.D. 7B) whereas hydrogen peroxide which generateshydroxide radical was detrimental to the selectivity improvement (SampleI.D. 8B).

Example 8

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose Bengal in combination with hydrogen peroxide.The amount of MgSO₄ added for sample I.D. #8A and 8B was 0.009 g. Theamount of H₂O₂ added for sample I.D. #8A and 8B was 0.6437 g.

TABLE 8 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER - H₂O₂Addition. Mw Intrinsic (g/mol) Selectivity Selectivity Sample ViscosityKappa SEC Selectivity Selectivity Improvement Improvement TAPPI I.D.Description [η], mL/g Number (10⁵) from [η] from Mw from [η], % from Mw,% Brightness 8A Molecular 603 22.4 928,000 26.9 41,400 — — 27.7 Oxygen8B Singlet 602 22.3 856,000 27.0 38,400 0.37 −7.25 28.6 Oxygen

Example 9

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose bengal. As light source, two Craftsman® 23 Wfluorescent work lights were used. The amount of MgSO₄ added for sampleI.D. #9A and 9B was 0.009 g.

TABLE 9 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER -Fluorescent Light. Intrinsic Mw Selectivity Selectivity Sample ViscosityKappa (g/mol) Selectivity Selectivity Improvement Improvement TAPPI I.D.Description [η], mL/g Number SEC from [η] from Mw from [η], % from Mw, %Brightness 9A Molecular 607 24.9 842,000 24.4 33,800 — — 26.8 Oxygen 9BSinglet 579 23.2 883,000 24.9 38,100 2.05 15.1 27.2 Oxygen

The data in Table 9 illustrates that fluorescent lamp can also be usedas a light source to generate singlet oxygen.

Example 10

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose bengal. Unbleached, prehydrolyzed krafthardwood (from Appleton Paper Inc., Roaring Springs, Pa.—40-45% RED OAK,10-15% WHITE OAK along with popular and ash) pulp was used as receivedafter drying. Its starting Kappa number was 15.2, its starting intrinsicviscosity was 668 mL/g, and its weight-average molecular weight was1,080,000 daltons. The amount of MgSO₄ added for sample I.D. #10A and10B was 0.009 g.

TABLE 10 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON HARDWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER. IntrinsicMw Selectivity Selectivity Sample Viscosity Kappa (g/mol) SelectivitySelectivity Improvement Improvement TAPPI I.D. Description [η], mL/gNumber SEC from [η] from Mw from [η], % from Mw, % Brightness 10AMolecular 616 10.8 1,000,000 57.0 92,600 — — 35.6 Oxygen 10B Singlet 62810.5 1,060,000 59.8 10,100 4.91 9.03 36.6 Oxygen

The data in Table 10 demonstrates enhanced selectivity improvement inoxygen delignification and bleaching of hardwood. The selectivityimprovement is not as large as with softwood because hardwood containsless lignin than softwood. A selectivity improvement of 9.03% (SampleI.D. 10B) based on the ratio of SEC Mw versus Kappa number is obtained.

Example 11

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose bengal. Unbleached, prehydrolyzed mechanicalpulp was used as received after drying. Its starting Kappa number was57.9, its weight-average molecular weight was 689,000 daltons, and TAPPIbrightness 47.6. The amount of MgSO₄ added for sample I.D. #11A and 11Bwas 0.009 g.

TABLE 11 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON MECHANICAL PULP USING ROSE BENGAL AS THE PHOTOSENSITIZER.Selectivity Sample Kappa Mw (g/mol) Selectivity from Improvement TAPPII.D. Description Number SEC Mw from Mw, % Brightness 11A Molecular 67.6621,000  9,186 — 58.6 Oxygen 11B Singlet 52.0 523,000 10,058 9.49 56.1Oxygen

The data set forth in Table 11 uses mechanical pulp as opposed tochemical pulp used in Examples 1-10 and Example 12. Table 11 illustratesthe selectivity improvement of oxygen delignification with singletoxygen on mechanical pulp compared to the results obtained withmolecular oxygen. Sample I.D. #11B shows 9.49% selectivity improvement(based on the ratio of SEC Mw versus Kappa number) with singlet oxygenwas obtained. The intrinsic viscosity of Sample I.D. #11A and 11B wasnot obtained due to the fact that the samples would not completelydissolve in solution.

Example 12

In this example, the Parr reactor was charged as described in Example 2.The pressure manifold was prepared as described in Example 2 except thephotosensitizer was rose bengal and the consistency was varied-25% highconsistency, sample I.D. #12A and 12B; 10% medium consistency, sampleI.D. #12C and 12D; and 5% low consistency, sample I.D. #12E and 12F. Theamount of MgSO₄ added for sample I.D. #12A-12F was 0.009 g.

TABLE 12 EFFECT OF HIGH PRESSURE OF MOLECULAR OXYGEN VERSUS SINGLETOXYGEN ON SOFTWOOD USING ROSE BENGAL AS THE PHOTOSENSITIZER AT VARIOUSCONSISTENCIES. Intrinsic Mw Selectivity Sample Viscosity Kappa (g/mol)Selectivity Selectivity Selectivity Improvement TAPPI I.D. Description[η], mL/g Number SEC from [η] from Mw Improvement, % from Mw, %Brightness 12A Molecular 572 23.2 853,000 24.7 36,800 — — 26.3 Oxygen12B Singlet 536 22.7 821,000 23.6 36,200 −4.2 −1.63 26.2 Oxygen 12CMolecular 607 24.9 884,000 24.4 35,500 — — 26.8 Oxygen 12D Singlet 71121.3 930,000 33.4 43,700 36.9 23.0 26.9 Oxygen 12E Molecular 586 22.1950,000 26.5 43,000 — — 27.9 oxygen 12F Singlet 592 22.2 874,000 26.739,400 0.75 −7.0 29.6 Oxygen

The data in Table 12 demonstrate the enhanced selectivity improvementwith singlet oxygen at various consistencies. The best selectivityimprovement with singlet oxygen was obtained at medium consistency, 10%.

1. A process for the delignification or bleaching of lignocellulosicpulp comprising the steps of a) providing an aqueous or non-aqueousmixture of pulp, b) providing a stream of molecular oxygen, reacting themolecular oxygen in the presence of visible light and at least onechemical agent photochemically converting a portion of the molecularoxygen to singlet oxygen and c) subsequently contacting pulp or pulpmixture with the singlet oxygen stream.
 2. The process of claim 1wherein the chemical agent comprises at least one photosensitizer dye.3. The process of claim 2 wherein at least one photosensitizer dye isselected from the group consisting of methylene blue and rose bengal orcombination thereof.
 4. The process of claim 1 wherein at least 10% ofthe molecular oxygen is converted to singlet oxygen.
 5. The process ofclaim 1 wherein the mixture of pulp is at a consistency of 14% or less.6. The process of claim 1 wherein the mixture of pulp is at atemperature of from about 20° C. to 160° C.
 7. The process of claim 1wherein the selectivity improvement is at least 9% based on molecularweight.
 8. A process for the delignification or bleaching oflignocellulosic pulp comprising the steps of: a) providing an aqueous ornon-aqueous mixture of pulp under typical oxygen delignificationconditions, b) separately providing a stream of molecular oxygen,reacting the molecular oxygen light in the presence of visible light anda chemical agent, photochemically converting a portion of the molecularoxygen to singlet oxygen, and c) subsequently contacting pulp or pulpmixture with the singlet oxygen stream.
 9. The process of claim 8wherein the chemical agent comprises at least one photosensitizer dye.10. The process of claim 8 wherein at least 10% of the molecular oxygenis converted to singlet oxygen.
 11. The process of claim 8 wherein themixture of pulp is at a consistency of 14% or less.
 12. The process ofclaim 8 wherein the mixture of pulp is at a temperature of from about80° C. to 150° C.
 13. The process of claim 8 wherein the mixture of pulpis at a pressure of at least 0.1 MPa.
 14. The process of claim 8 whereinthe mixture of pulp is at a pH of at least about 8.5 or higher.
 15. Theprocess of 8 wherein the singlet oxygen is added to one of more stagesof an oxygen delignification process.
 16. A process for thedelignification or bleaching of lignocellulosic pulp comprising thesteps of: a) providing an aqueous or non-aqueous mixture of mechanicalpulp under typical mechanical pulping conditions, b) separatelyproviding a stream of molecular oxygen, reacting the molecular oxygen inthe presence of visible light and a chemical agent, photochemicallyconverting a portion of the molecular oxygen to singlet oxygen and c)subsequently contacting pulp or pulp mixture with the singlet oxygenstream.
 17. The process of claim 16 wherein the chemical agent comprisesat least one photosensitizer dye.
 18. The process of claim 16 wherein atleast 10% of the molecular oxygen is converted to singlet oxygen. 19.The process of claim 16 wherein the mixture of pulp is at a consistencyof 14% or less.
 20. The process of claim 16 wherein the singlet oxygenis applies to the mechanical pulp during the pulping process at the eyeof a mechanical pulp refiner or through the dilution water for stonegroundwood.
 21. A method to generate singlet oxygen that comprises thesteps of providing a molecular oxygen source, a visible light source, achemical agent wherein the molecular oxygen source provides a stream ofmolecular oxygen into a chamber or a series of chambers containing thechemical agent, contacting the molecular oxygen with the chemical agentin the presence of visible light from the light source.
 22. The processof claim 21 wherein the chemical agent comprises at least onephotosensitizer dye.
 23. The process of claim 21 wherein the chambercontains the chemical agent wherein the chemical agent is affixed to gasfilters, glass beads, wire mesh, or a catalytic bed.
 24. A pulp or paperproduct produced by the process of claim 1.